small-angle approximation

The specific linearization of trigonometric functions near `θ = 0`: `sin θ ≈ θ`, `cos θ ≈ 1`, `tan θ ≈ θ`, with `θ` in radians. At `θ = 5°` (≈ 0.087 rad) the error in `sin θ ≈ θ` is about 0.13%; at `θ = 30°` (≈ 0.52 rad) it's about 4.5%; at `θ = 60°` (≈ 1.05 rad) it's nearly 18%. The approximation turns the pendulum equation `θ̈ = −(g/L) sin θ` into the harmonic-oscillator equation `θ̈ = −(g/L) θ`, whose solutions everyone knows.